Nonaqueous electrolyte batteries which are charged/discharged based on the movement of lithium ions between a negative electrode and a positive electrode are being investigated and developed as batteries having a high energy density. Nonaqueous electrolyte batteries including a lithium transition metal composite oxide and a carbonaceous material as active materials have already been commercialized. In the lithium transition metal composite oxide, Co, Mn, Ni, or the like is used as a transition metal.
Recently, nonaqueous electrolyte batteries including, for example, a lithium titanium composite oxide (about 1.5 V vs. Li/Li+) having a lithium absorption-release potential higher than that of the carbonaceous material as an active material have been investigated (see Jpn. Pat. No. 3866740 and JP-A 9-199179 (KOKAI)). Since the lithium titanium composite oxide changes little in volume with charge and discharge, it is excellent in cycle characteristics. Such nonaqueous electrolyte batteries are theoretically free from lithium metal deposition and can be charged at a high current.
However, the amount of water adsorbed on the surface of the lithium titanium composite oxide is large, and a hydroxyl group is present on the surface. Thus, water mainly reacts with the nonaqueous electrolyte to produce a free acid. The produced free acid dissolves an active material of a positive electrode. The dissolved substance is precipitated on the surface of a negative electrode, resulting in an increase in surface resistance. Consequently, the battery capacity, particularly the life performance accompanied by the resistance increase is reduced.